This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Human centrin 2 (20 kDa) and calbindin D28K (30 kDa) are members of the family of Ca2+-binding proteins. We use PLIMSTEX, the study of Protein Ligand Interactions by Mass Spectrometry, Titration, and H/D Exchange, to explore the calcium binding properties of these proteins. This strategy can show the conformational change upon binding calcium, the affinity constants, and also the binding stoichiometry.